Baluns are circuit elements that provide balance-unbalance transformation and suppress common mode currents. Existing baluns are complicated, work for only one or two closely related channels, and are rarely efficient at high power. Existing baluns are of several types and have a variety of drawbacks.
Baluns consisting of discrete transmission lines, such as (a) The Quarter Wavelength Sleeve Balun [1. Y. L. Chow, K. F. Tsang, C. N. Wong, An Accurate Method To Measure The Antenna Impedance of A Portable Radio, Microwave and Optical Technology Letters, Volume 23 Issue 6, Pages 349-352, 1999], (b) The Half-Wavelength Balun [2. Modern Antenna Design, Second Edition, Thomas A. Milligan, ISBN10: 0471457760, John Wiley, 2005], and (c) the Marchand balun [RF Design Guides: Systems, Circuits and Equations, Peter Vizmuller, ISBN: 0-089006-754-6, Artech House, Inc., 1995; Rutkowski, T. Zieniutycz, W. Joachimowski, K. Gdansk Div., Wideband Coaxial Balun For Antenna Application, Microwaves and Radar, 1998. MIKON '98., 12th International Conference on, Volume 2, Pages 389-392, ISBN: 83-906662-0-0, 1998], are bulky and long, and are difficult to build and adjust because they require precise machining.
Transformer type baluns that contain ferrite cores or beads [Onizuka Masahiro, Sato Kouki, Balun Transformer Core Material, Balun Transformer Core and Balun Transformer, U.S. Pat. No. 6,217,790, 2001] are lossy, not suitable for very high power, and not suitable in magnetic fields (as in NMR and MRI). They are also subject to heating problems, saturation problems and stray couplings.
The air-core transformer type balun [Weiss Michel, Martinache Laurent, Gonella Olivier, Multifrequency Power Circuit and Probe and Spectrometer Comprising Such A Circuit, U.S. Pat. No. 7,135,866, 2006], needs precise alignment, is dependent on the resonance tuning of peripheral parts, and is subject to stray coupling.
Ferrite choke type baluns [Werlau Glenn, High Power Wideband Balun And Power Combiner/Divider Incorporating Such A Balun, U.S. Pat. No. 6,750,752, 2004] are lossy, not suitable for very high power, not suitable in magnetic fields (as in NMR and MRI) and subject to heating problems.
Air-core choke baluns [Burl Michael, Chmielewski Thomas, Braum William O., Multi-Channel Balun For Magnetic Resonance Apparatus, U.S. Pat. No. 6,320,385, 2001; Harrison William H., Arakawa Mitsuaki, Mccarten Barry M., RF Coil Coupling For MRI With Tuned RF Rejection Circuit Using Coax Shield Choke, U.S. Pat. No. 4,682,125, 1987] require an excessively large bending radius in the thick transmission lines required to handle very high power.
Transistor circuit baluns [Lee Young Jae, Yu Hyun Kyu, Active Balun Device, U.S. Pat. No. 7,420,423, 2008] are lossy, temperature sensitive, noisy and not suitable for high power applications.
Stripe line baluns, made from printed circuit board or laminate, [Niu Dow-chih, Chang Chi-yang, Lin Lih-shiang, Balun-Transformer, U.S. Pat. No. 6,531,943], are lossy, fragile, temperature sensitive, and not suitable for high power applications.
The dual band balun, comprising discrete transmission lines which can balance two working frequencies, [Clemens Icheln, Joonas Krogerus, and Pertti Vainikainen, Use of Balun Chokes in Small-Antenna Radiation Measurements, IEEE Transactions on Instrumentation and Measurement, Vol. 53, No. 2, pp. 498-506, 2004] has a mechanical tuning low pass filter that needs precise machining. Balancing the higher frequency requires changing the length of the balun. Furthermore, the two frequencies are closely related and cannot be adjusted independently. All of the above are incorporated by reference herein.
In some application such as communication antennas (including radio, television, wireless, and cell), common mode currents cause power loss, noise pick-up, and safety hazards. Baluns can improve efficiency and safety by suppressing the common mode currents. Multi-frequency baluns would allow antennas and other devices to operate efficiently and safely at multiple frequencies.
Nuclear magnetic resonance (NMR) spectroscopy (including magnetic resonance imaging—MRI) detects radio-frequency (RF) transitions between nuclear spin states. This requires delivery and detection of radio-frequency radiation by a coil around the sample. For multi-nuclear magnetic resonance, the coil must operate at multiple, disparate frequencies. And, to work well, it must be balanced at all these frequencies.
Sample coil imbalance reduces the homogeneity of the radiation, and thereby reduces excitation efficiency. Sample coil imbalance also causes signal loss and noise pick-up, resulting in poor signal-to-noise ratio. At high power, such as is required in solid state NMR, sample coil imbalance increases sample heating and arcing. Sample coil imbalance also compromises tuning and matching for salty or high dielectric samples. All of these effects of coil imbalance are greatly exacerbated at the high fields preferred in modern magnetic resonance spectroscopy.
Existing balanced NMR probes are either not fully transmission line or are balanced over only a narrow frequency range. By avoiding lump circuit elements, fully transmission line magnetic resonance probes achieve high efficiencies, reduced cross-talk between channels, and robust operation across a wide range of temperatures. Fully transmission line probes have the further advantages that (a) all the controls are in the bottom box which is outside the magnet and therefore accessible and always at room temperature, and (b) improved isolation between channels is possible through the design of common null points. However, in these probes, it is difficult to balance multiple channels at significantly different frequencies. A further challenge is conforming a fully transmission line probe to the dimensions of the NMR magnet and the associated facility, while maintaining balance, impedance matching and efficiency, especially over a multi-band (multi-frequency) operating range.